This invention relates to the field of gas pressure relief valves, and, in particular, it relates to a gas pressure relief valve for use between a gas pump or compressor and pneumatic components of a medical ventilator or similar equipment, whereby the pressure delivered to the pneumatic components remains substantially constant throughout a wide range of gas flow rates.
Typically, overpressure relief in a pneumatic system employing an air or gas compressor is provided by a poppet-type valve, which is spring-biased to open when a preselected pressure is exceeded. While in many applications a poppet-type valve achieves satisfactory results, the inherent limitations in poppet-type valves make them less than optimal for use in certain other applications. Specifically, poppet valves are usually operable in only a relatively narrow flow rate range without the introduction of relatively large pressure changes in the system due to the increasing force applied by the bias spring as the valve opens more widely to accommodate increasing flow rates. This factor may limit the utility of poppet valves in applications where precise pressure regulation is required over a wide range of fluid flow rates.
One particular application in which precise pressure regulation over a wide range of fluid flow rates is important is that of medical ventilators, used in assisting the breathing action of patients with pulmonary disabilities. Such devices typically employ an air compressor or pump to supply breathing gas (usually a variable mixture of air and pure oxygen) to a patient at flow rates which depend in large measure on the demands of the patient. These flow rates for a typical patient can range from near zero up to approximately 120 liters per minute (LPM). Moreover, such devices typically employ complex pneumatic components in controlling the flow of gas in accordance with the patient's needs. The accuracy and efficiency of such components can be increased, and their cost lowered, if the pressure supplied to them by the air compressor is precisely regulated so as to remain substantially constant, within close limits, over the full range of expected flow rates. Furthermore, constant pressure operation is desirable from the standpoint of the compressor, inasmuch as substantially constant load requirements allow the use of a smaller, and therefore less expensive, compressor than if varying loads are imposed.
Heretofore, in medical ventilators, overpressure relief between the compressor and the pneumatic components of the system has been achieved primarily by the use of poppet-type valves. See, for example, U.S. Pat. No. 3,756,229 to Ollivier. In order for such valves to perform acceptably over wide flow rate ranges, they must be carefully engineered, having, for example, springs specifically engineered (as to size, spring rate, etc.) to minimize, as much as possible, the effects of widely varying flow rates. Even so, variations from the nominal regulated pressure of 10 percent to 15 percent are typical for such valves when they are subjected to a flow rate range of 0 to 120 LPM.
Accordingly, a means has been sought for providing overpressure relief between the compressor and the pneumatic circuitry in devices such as medical ventilators wherein the aforementioned limitations imposed by poppet-type relief valves are avoided, or at least minimized, thereby allowing the delivery of a substantially constant gas pressure to the pneumatic circuitry over a wide range of flow rates.